25 Besides, the Church demands from those who have devoted themselves to furthering her interests, something very different from the dwelling upon profitless questions; she demands that they should devote the whole of their energy to preserve the faith intact and unsullied by any breath of error, and follow most closely him whom Christ has appointed to be the guardian and interpreter of the truth. There are to be found today, and in no small numbers, men, of whom the Apostle says that: "having itching ears, they will not endure sound doctrine: but according to their own desires they will heap up to themselves teachers, and will indeed turn away their hearing from the truth, but will be turned unto fables" (II Tim . iv. 34). Infatuated and carried away by a lofty idea of the human intellect, by which God's good gift has certainly made incredible progress in the study of nature, confident in their own judgment, and contemptuous of the authority of the Church, they have reached such a degree of rashness as not to hesitate to measure by the standard of their own mind even the hidden things of God and all that God has revealed to men. Hence arose the monstrous errors of "Modernism," which Our Predecessor rightly declared to be "the synthesis of all heresies," and solemnly condemned. We hereby renew that condemnation in all its fulness, Venerable Brethren, and as the plague is not yet entirely stamped out, but lurks here and there in hidden places, We exhort all to be carefully here and there in hidden places, We exhort all to be carefully on their guard against any contagion of the evil, to which we may apply the words Job used in other circumstances: "It is a fire that devoureth even to destruction, and rooteth up all things that spring" ( Job xxxi. 12). Nor do We merely desire that Catholics should shrink from the errors of Modernism, but also from the tendencies or what is called the spirit of Modernism. Those who are infected by that spirit develop a keen dislike for all that savours of antiquity and become eager searchers after novelties in everything: in the way in which they carry out religious functions, in the ruling of Catholic institutions, and even in private exercises of piety. Therefore it is Our will that the law of our forefathers should still be held sacred: "Let there be no innovation; keep to what has been handed down." In matters of faith that must be inviolably adhered to as the law; it may however also serve as a guide even in matters subject to change, but even in such cases the rule would hold: "Old things, but in a new way."
Source: Ad Beatissimi Apostolorum (Vatican.va)